The present invention relates generally as indicated to a hydraulic motor and, more particularly, to a hydraulic motor with a gerotor drive assembly which provides rotational motion to a desired piece of machinery.
A hydraulic motor is a converter of pressurized oil flow into torque and speed for transferring rotational motion to a desired piece of machinery. Of particular relevance to the present invention is a hydraulic motor, wherein this conversion is accomplished by a drive assembly having a gerotor set. A gerotor motor can provide a combination of compact size, low manufacturing cost, and high torque capacity, thereby making it a very popular choice for heavy duty applications requiring low speeds (e.g., 1000 rpm or less) and high torques (e.g., 15,000 In-Lb or more).
A gerotor set comprises an outer stator and an inner rotor having different centers with a fixed eccentricity. The stator has internal teeth or xe2x80x9cvanesxe2x80x9d which form circular arcs, and the inner rotor has one less external xe2x80x9cteethxe2x80x9d or lobes. The rotor lobes remain in contact with the circular arcs as the rotor moves relative to the stator, and these continuous multi-location contacts create fluid pockets which sequentially expand and contract. As fluid is supplied and exhausted from the fluid pockets in a timed relationship, the rotor moves hypocycloidally (i.e., orbits and rotates) relative to the stator.
A drive link is interconnected to the rotor for movement therewith, and this interconnection usually constitutes crowned external splines on the drive link which engage with internal splines on the rotor. Such a splined mating arrangement allows the drive link to xe2x80x9cwobblexe2x80x9d during operation of the motor. To prevent the drive link from slipping axially backward out of the splined engagement, an axial stop can be provided adjacent the rear end (or nose portion) of the drive link.
The drive assembly of a gerotor motor will typically include a valving system to supply and exhaust the fluid from the gerotor pockets in the desired timed relationship. One common type of valving system includes a disk-type commutator and a stationary valve member (e.g., a manifold). A slow-speed commutator rotates at the speed of rotation of the rotor, and manifold channels are opened/closed in the angular circumferential direction using edges of the valve openings. A fast-speed commutator orbits with the rotor and the commutator""s inner diameter and outer diameter control fluid metering. Generally, a fast-speed commutator is preferred because it allows valving to be synchronized with the volume changes of the gerotor fluid pockets (rather than rotation of the shaft), thereby significantly reducing timing errors.
The use of a commutator creates the potential for cross-port leakage (e.g., flow bypasses the drive assembly) at the interface between the commutator and an end cover. To prevent such cross-port leakage, a groove can be formed in the back axial face of the commutator and a triangular or trapezoidal (in cross-section) sealing ring positioned therein. The sealing ring is usually oversized (e.g., the height of the ring is greater than the depth of the groove) so that, when the motor is at rest, the ring projects outwardly from the groove. Upon start-up of the motor, the hydraulic imbalance pushes the sealing ring out of the groove to perform the sealing at the interface between the commutator and end cover.
The drive link is interconnected to a shaft to transfer rotational movement thereto. For example, the motor can include a coupling shaft which is connected to the drive link (e.g., by a splined interconnection) and which can be coupled to the input shaft of the desired piece of machinery. In this case, the drive assembly (e.g., the commutator, the manifold and the gerotor set) is commonly positioned between the motor""s end cover and a housing which rotatably supports the coupling shaft. Alternatively, the shaft can be part of the gearbox of the desired machinery and the drive link is directly coupled thereto. In this case, the drive assembly is commonly positioned between the motor""s end cover and a mountable housing for attachment to the gearbox. In either case, a plurality of bolts extend through registered openings in the end cover, the drive assembly and the housing to clamp these components together. A wear plate can be positioned between the drive assembly and the housing, and the clamping bolts can also extend therethrough. Face seals are provided between the various components to prevent leakage at the interfaces.
A hydraulic motor will have a flow circuit which determines the path of fluid flow and can be viewed as defining a cylindrical pressure vessel. The diameter of the pressure vessel is determined by the outermost radial reach of the fluid circuit, and the length of the pressure vessel is determined by the longest axial reach of the fluid circuit.
The flow circuit of a hydraulic motor includes a working path which extends between the inlet port and the outlet port and through which the fluid passes to cause the drive assembly to rotate the output shaft in the appropriate direction. When the motor is operating in a first direction, the first port is the inlet port and the second port is the outlet port and the output shaft rotates in a first direction (e.g., clockwise). When the motor is operating in a second direction, the second port is the inlet port and the first port is the outlet port and the output shaft rotates in a second direction (e.g., counterclockwise). In either case, the inlet port can be connected to a pump discharge and the outlet port can be connected to a return line to a reservoir which feeds the pump suction.
In most hydraulic motor designs, the working path extends through non-working portions of the motor (e.g., the housing and/or an axial passageway in the drive link), whereby the length of the working path extends for a substantial distance of the pressure vessel. Also, most hydraulic motors have a xe2x80x9cwet boltxe2x80x9d design, wherein the clamping-bolt openings double as fluid passageways and face seals are located radially outside the diameter of the circular array of clamping bolts. This arrangement results in the diameter of the pressure vessel occupying a substantial portion of the motor""s radial dimension, and requires the clamping bolts to directly absorb corresponding forces.
The flow circuit of a hydraulic motor will usually also include a non-working path, including chambers surrounding the drive train components (i.e., the drive link and the coupling shaft) and through which fluid passes for cooling and lubrication of these components. In a two-pressure-zone motor design, fluid traveling through the non-working path rejoins fluid traveling through the working path somewhere upstream of the outlet port. In a three-pressure-zone motor design, fluid traveling through the non-working path does not rejoin the working path and exits the motor through a separate case drain in the housing.
A three-pressure-zone motor design is used in applications where contamination flushing must be performed. Additionally or alternatively, a three-pressure-zone design is used for applications in which the drive link is coupled directly to the input shaft of a gearbox. Otherwise, a two-pressure-zone motor design usually is employed because it simplifies plumbing criteria, reduces reservoir size requirements, decreases pump capacity demands, and minimizes the risk of xe2x80x9cdead zonesxe2x80x9d within the motor.
Some of the most significant considerations when selecting a hydraulic motor, especially for heavy-duty applications, include the motor""s no-load pressure drop (or mechanical efficiency), its life expectancy, its start-up (or breakaway) efficiency, and/or its torque capacity. Accordingly, motor manufacturers are constantly trying to improve upon these performance parameters.
The present invention provides a hydraulic motor which, when compared to conventional hydraulic motors, can be constructed to have an improved no-load pressure drop, a longer life expectancy, a better start-up efficiency and/or a higher torque capacity. The motor can be especially well suited for heavy-duty applications requiring low speeds and high torques.
More particularly, the present invention provides a hydraulic motor comprising an end cover, a drive link, a drive assembly, and a flow circuit extending between a first port and a second port. The flow circuit comprises a working path through which fluid flows to cause the drive assembly to hypocycloidally move the drive link in a first direction when the first port is the inlet port and in a second direction when the second port is the inlet port. When the motor is operating in a first direction, the fluid flows in a first direction through the working path of the fluid circuit and, when the motor is operating in a second direction, the fluid flows in a second direction through the working path of the fluid circuit. The motor can be designed to operate in only one direction (either the first or the second) or can be designed to operate in both directions. The flow circuit can also comprise a non-working path passing through chambers surrounding the drive link to cool and lubricate the drive train components.
According to one aspect of the invention, the first port and the second port are part of the end cover, and the working path is axially confined to a length between the end cover and the drive assembly. As such, the working fluid is not subjected to no-load pressure drops from unnecessary travel through non-working portions of the motor. This confinement of the working path results in a significantly reduced pressure drop (e.g., 50% less) when compared to conventional hydraulic motors of similar size and/or capacity and this translates into a dramatic improvement in motor efficiency.
According to another aspect of the invention, the clamping bolts are radially positioned outside of the motor""s pressure vessel and, in any event, they do not communicate with any of the motor""s fluid chambers. This radially outward positioning of the clamping bolts, or xe2x80x9cdry boltxe2x80x9d design, results in less axial tensile stress per bolt for a motor design having a given number of clamping bolts. Additionally or alternatively, because fluid flow characteristics do not play a part in bolt placement, more clamping bolts can be used in a given motor design. Less strain-per-bolt and/or more bolts-per-motor result in less bolt-stretching and equal bi-directional motor performance which, in turn, results in a longer motor life. Furthermore, this xe2x80x9cdry boltxe2x80x9d design avoids the extra manufacturing cost of countersink machining which is required in a xe2x80x9cwet boltxe2x80x9d design.
According to another aspect of the invention, a non-interference seal arrangement is used at the valving interface between the end cover and the drive assembly. In this arrangement, a sealing ring is positioned in a groove in the commutator. The height of the sealing ring is less than the depth of the groove, whereby the seal does not project outwardly from the groove when the motor is at rest. Also, the groove and seal can each have a roughly rectangular cross-sectional shape such that the ring resides loosely within the groove when the motor is at rest and then, upon start-up of the motor, is appropriately moved to a position which prevents cross-port leakage. Specifically, the seal is pushed rearward by hydraulic imbalance forces and is pushed in the appropriate radial direction by the port-to-port pressure differential. With an oversized seal, mechanical friction is created between the seal and the end cover during startup or very slow speed operation (e.g., 10 rpm or less). With the sealing arrangement of the present invention, this mechanical friction is eliminated thereby enhancing start-up and low speed efficiency and increasing the life of the sealing ring.
According to a further aspect of the invention, an axial stop for the drive link is mounted on a moving part of the drive assembly and, more particularly, is preassembled on an internal diameter of the rotor. When the axial stop is mounted on a stationary component of the motor (e.g., the end cover), the drive link will rotate/orbit relative to the axial stop, thereby creating internal mechanical friction therebetween. However, with the axial stop system of the present invention, this internal friction is eliminated, thereby improving the motor""s startup efficiency.
According to a further aspect of the invention, the drive link has an axial passageway which allows a component of the drive train (e.g., a coupling shaft) to centrifugally pump a diverted portion of fluid from the working path through the non-working path. Regardless of whether the motor is operating in the first direction or the second direction, the diverted portion of the fluid is centrifugally pumped through the non-working path in the same direction by the output shaft. When the motor is operating in the first direction, the non-working portion of the fluid is diverted from the high pressure (pre-working) fluid and, when the motor is operating in the second direction, the non-working portion of the fluid is diverted from the low pressure (post-worked) fluid. This non-working path is believed to provide superior lubrication for the splined interconnection between the drive link and the rotor and/or the splined interconnection between the drive link and the output shaft. Since, in general, the torque capacity of a motor is limited by the condition of its drive train components, this superior lubrication arrangement can greatly enhance the performance of a motor. This aspect of the invention finds particular application in two-pressure-zone motor designs but can also be used in three-pressure-zone motor designs as well.
These and other features of the invention are fully described and particularly pointed out in the claims. The following description and drawings set forth in detail certain illustrative embodiments of the invention, these embodiments being indicative of but a few of the various ways in which the principles of the invention may be employed.